1. Field of the Invention
The present invention relates to a method of controlling a charging potential of a charging mechanism having a conductive roller in a printer, and more particularly, to a method of controlling a charging potential of a conductive roller by using a sensing resistance in a printer.
2. Description of the Related Art
A printer generally includes an organic photoconductive cell (OPC), a discharging mechanism eliminating a potential of the OPC, a charging mechanism increasing the potential of the OPC to a charging potential, an exposure mechanism radiating a beam on the OPC to form an electrostatic latent image, a development mechanism supplying a developing solution to the OPC to develop the electrostatic latent image, a drying mechanism drying an image formed on the OPC, and a transfer mechanism transferring the image on the OPC to a sheet.
The charging mechanism supplies a predetermined charging voltage to the OPC after the OPC is discharged, so as to increase the potential of the OPC to a predetermined charging potential level. Here, if a charging characteristic of the OPC is changed due to continuous use of the printer, a residual potential of the OPC increases, and thus the charging potential of the OPC does not increase in proportion to the supplied charging voltage. When the charging potential of the OPC does not increase to the predetermined level, a difference between the charging potential of the OPC and an exposure potential of the exposure mechanism or the charging potential of the OPC and a development potential of the development mechanism decreases so that a desired image cannot be printed.
Generally, a resistance of a conductive roller of the charging mechanism may increase as much as about ten times according to changes in temperature and moisture, and thus the charging potential of the OPC seriously fluctuates. When the temperature and the moisture are low, and the charging potential of the OPC is also low, contamination may occur in a non-image region of the sheet. When the temperature and the moisture are high, and the charging potential of the OPC is also high, a printing quality of an output image is lowered.
Accordingly, it is necessary to control the charging potential of the OPC to be within a predetermined range.
FIGS. 1 and 2 are schematic views illustrating conventional methods of controlling a charging potential of an OPC 13 by using a conductive roller 11 in a conventional charging mechanism.
FIG. 1 is a schematic view illustrating the conventional method of controlling the charging potential of the OPC 13 by using a surface electrometer.
In order to charge the OPC 13 to a predetermined potential level, an engine controller unit (ECU) 21 outputs a voltage signal to a high voltage power supply (HVPS) 23, and the HVPS 23 receives the voltage signal and applies a high voltage of about 700 to 1500 V to a metal shaft of the conductive roller 11. Accordingly, a strong electric field is formed between a surface of the conductive roller 11 and the OPC 13 so that a Townsend discharge occurs, and corona ions accumulate in the OPC 13 to charge the OPC 13.
As a printing operation is performed, the potential of the OPC 13 is varied to print images. Here, the charging potential of the OPC 13 cannot be maintained to be uniform due to changes in internal and external environments. Since the changes in the charging potential of the OPC 13 may cause deterioration of the printing quality of the output image, it is required to maintain the charging potential within a tolerance range.
The conventional method of controlling a charging potential of FIG. 1 detects the charging potential by using a surface electrometer 15 located on a surface of the OPC 13 and outputs an analog signal about the detected charging potential to a sensor board 17. Thereafter, an analog-to-digital converter (ADC) 19 converts the analog signal into a digital signal. The ECU 21 receives the digital signal and establishes a new target charging voltage considering a difference between the detected charging potential and a target potential and outputs an adjusted voltage signal to the HVPS 23 so as to control the charging voltage of the conductive roller 11.
FIG. 2 is a schematic view illustrating another conventional method of controlling the charging potential of the OPC 13 by using a sensing resistance.
Referring to FIG. 2, a sensing resistor 25 outputs a charging current signal in proportion to the charging potential of the OPC 13. An operational (OP) amplifier 27 amplifies the charging current signal and outputs the amplified signal to the ECU 21. Thereafter, the ECU 21 outputs a charging voltage signal to control the HVPS 23 in response to a difference between the amplified charging current signal and a target charging potential so that the HVPS 23 applies a high voltage to a conductive roller 11.
Since the conventional method of using the surface electrometer requires a separate surface electrometer, a price of the printer increases. In addition, only the charging potential is measured by the surface electrometer so that an electrical characteristic of the OPC, i.e., an increase of a residual potential, cannot be measured. Consequently, the charging potential of the OPC cannot be precisely controlled.
The conventional method of using the sensing resistance may compensate for a variation of the resistance of the conductive roller when a charging current is maintained. However, the conventional method cannot compensate for the variation of the electrical characteristic of the OPC, i.e., the variation of the charging characteristic due to changes in the residual potential.